152 ON THE PROCESSES OF 



tions they possess are carried on, and by what means they respectively ac- 

 quire maturity and perfection. 



But it is not only necessary that the system should in this manner be ma- 

 tured and perfected by a fresh application of materials, but that the old mate- 

 rials which constitute every organ should be progressively removed from the 

 system, in consequence of their being worn out by use, and their place sup- 

 plied from definite stores. Let us, then, devote the present hour to an 

 inquiry how this latter change occurs in vascular and living matter, in the 

 vegetable and animal system : by what means the dead or exhausted and 

 worn-out elements of the different organs are carried off, and replaced by new 

 reformative materials, and what are the principal phenomena that result from 

 such a series of operations. 



The blood, then, in animals, and the sap, which may be regarded as a spe- 

 cies of blood, in plants, of both which we have already treated, are the vital 

 currents from which every organ of the individual frame derives the nourish- 

 ment it stands in need of, and into which it pours ultimately a considerable 

 portion of its waste and eliminated fragments ; for the provident frugality of 

 nature suffers nothing to be lost, and, as far as possible, works up the old 

 materials, time after time, into fresh food for the subsistence of the entire 

 system. 



To produce this double purpose two distinct sets of vessels are necessary: 

 one for that of separating from the common mass of the blood, and recom 

 bining into new associations, those particular parts of it which the formation 

 of the fresh matter demands ; and the other for that of carrying back the 

 rejected materials into the general current. And hence these two sets of ves- 

 sels bear the same relation to each other as the veins and arteries of the ani- 

 mal frame, accompany every part of the frame to its farthest extremities, and, 

 indeed, constitute the general mass of the frame itself. From the respective 

 offices they perform, they are denominated SECERNENT and ABSORBENT sys- 

 tems : in their utmost ramifications they are too minute to be traced by the 

 keenest eye, or the nicest experiment of the anatomist; but where they are 

 not quite so minute, they are sufficiently discoverable, and their course is 

 sufficiently capable of being followed up, from the delicate apertures or 

 mouths by which, in infinite numbers, they open on all animal surfaces, or 

 hollows whatever, to their incipient sources. 



The SECERNENTS, or that set of vessels whose office it is to separate parti- 

 cular parts from the blood for particular purposes, are evidently continuations 

 of some of those very subtile ramifications of the arteries which, on account 

 of their fineness, are called capillary ; and the ABSORBENTS, or that set of ves- 

 sels whose office it is to imbibe or drink up the waste and exhausted materials, 

 are as evidently distinct and attenuate tubes, progressively uniting, and ulti- 

 mately emptying themselves into the venous system ; the common trunk in 

 which they concentre, and in which also concentre the lacteals of the ali- 

 mentary canal, named the thoracic duct, being a tough membranous channel, 

 situate upon the interior part of the spine, of about the diameter of a crow- 

 quill in man, and running in a serpentine direction through the diaphragm or 

 midriff to an angle formed by a union of the jugular and subclavian veins, 

 into which it opens, and where of course it terminates, leaving the waste and 

 the new food, now ultimately intermixed, to be still farther elaborated and 

 refitted for use by those subsequent and specific operations of the heart and 

 the lungs which we have already described.* 



The simplest action, perhaps, that is evinced by the mouths of the secre- 



* This double action by a double set of vessels was little, if at all, known to the ancients, who referred 

 the economy of both secretion and absorption to the powers of peculiar arteries and veins ; and hence, the 

 porosity of these vessels was a doctrine in common belief till the time of Hewson, Hunter, and Cruick- 

 shank. M. Magendie and M. Flandrin, of Paris, have of late been very active in establishing a view of 

 the subject in many respects not essentially different from that of the old school, and in teaching that the 

 only general absorbents are the veins; that the lacteals absorb food, but nothing else; and that the- 

 lymphatics have no absorbent power whatever. Their experiments are plausible and striking, but by no 

 means decisive enough to subvert the system explained above. The argument on both sides maybe found 

 in fthe author's Study of Medicine, vol. v. p. 278, 2d edit. 1825. 



